The present invention relates to pressure transmitters used to measure fluid pressures in industrial fluid processing plants. In particular, the present invention relates to the performance of such pressure transmitters in contaminated atmospheres that degrade the performance of electronic circuitry.
Electronic circuits in pressure transmitters are known to be sensitive to contamination from the atmospheres in industrial fluid processing plants. Such atmospheres can include humidity, fluorinated or chlorinated compounds and other contaminants that can degrade the performance of pressure transmitter circuitry. Sensitive silicon transistors or integrated circuits can be poisoned, microscopic bond wires can be corroded, or overall circuit performance can be degraded by leakage on circuit boards, all due to atmospheric contaminants.
Typically, silicon devices used for critical functions in pressure transmitters are protected in expensive hermetically sealed integrated circuit packages, such as an hermetic TO-5 metal can or hermetic ceramic dual-in-line package (DIP). Printed circuit boards in pressure transmitters are usually sprayed, brushed or dipped in a special moisture and fungus resistant varnish or conformal coating to reduce electrical leakage and damage due to contaminants, which is also expensive.
The transmitter circuit boards are then enclosed in a metal housing that has threaded covers and O-ring seals to provide an additional barrier to contamination. The threaded covers, however, need to be opened by technicians in the contaminated plant environment in order to make manual circuit adjustments such as adjusting calibration or setting configuration jumpers. When the threaded covers are replaced after adjustment, moisture and other contaminants can become trapped in the housings, and long term transmitter reliability is compromised.
A method of making a pressure transmitter, and a pressure transmitter are needed where the transmitter has an adjustable transmitter circuit that is protected from contaminated industrial atmospheres e without the use of expensive hermetic integrated circuit packages and special conformal coatings.
A pressure transmitter is disclosed with a sealed housing surrounding a cavity that is filled with a fill gas that is free of integrated circuit contaminants. A sensor circuit including an integrated circuit is placed in the cavity and a gas fill port on the housing is permanently sealed. The sensor circuit is electrically adjustable from outside the pressure transmitter and the transmitter""s housing does not need to be opened for adjustment. The integrated circuit is surrounded by the fill gas and protected from contaminated atmospheres present outside the transmitter.
In a preferred embodiment, the integrated circuit includes an economical non-hermetic integrated circuit package.